Thomas,
that has been discussed recently on eevblog, and another volt-nuts
stated, that the only difference was the implementation of a VHP101, 40k
resistor for R207. This information was supposed to come directly from
keysight, but has been deleted by the user.
The VHP101 has a T.C. < 0.3ppm/K, and is oil filled, so 2ppm/6yr drift
might be assumed, according to the Vishay datasheets.
This resistor was routinely assembled in the standard instruments since
about 1996, so 3458As younger than that should also be more stable,
although HPAK never adapted the specification. These resistors were all
marked by hand, i.e. an individual code on top of the housing, so a
selection may have taken part.
This VHP101 may improve the ACAL OHM T.C. and OHM 1-year-drift over the
original b.m.f. resistor, but the ACAL T.C. is not specified in the HFL
datasheet. The transfer accuracy also does not reflect correctly this
improvement, as the drift over 10min and 0.5°C for 10kOhm and 100kOhm
would only be related to the Ohm mode resistor array, RP300, which
obviously was not changed, and has a T.C. of about 3ppm/K, which would
yield a transfer stability of 1.5ppm for a 0.5°C change.
I think, the HFL specification is a hoax, especially, as nobody ever has
seen a real HFL instrument.
Frank
> Does anyone know how to identify a 3458A HFL A1 board?
>The HFL A9 is part number 03458-66529, but I have never seen a part
number for the A1 board with superior resistance measurement.
>And does the A1 use select parts like the A9 or different parts?
Thanks: Thomas Knox
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